Adhesives and cementitious compositions, such as, mortars, concretes, and plasters, typically comprise additives to improve one or more properties of the compositions. For example, the additive may improve certain rheological properties of the composition, such as sag resistance, or water retention properties of the composition, or adhesive properties of the compositions, or a combination of properties. For instance, it is taught that various carbohydrate polymers, such as cellulose ethers and starches, may be used independently in the cementitious and adhesive compositions as water retention aids. In mortars prepared specifically for adhering ceramic tiles to substrates, the water retention aid prevents the loss of water to the ceramic tile or substrate, allowing the use of a thin bed of mortar rather than a thick bed. The water retention aid also increases the amount of open time of the mortar, that is, the amount of time after the mortar is made but before it sets, and thus allows adjustment of the tile after it is initially placed in contact with the mortar composition.
While the noted polymers are generally known as water retention aids, modifications of the polymers are often made to improve rheological properties of the cementitious compositions containing the polymers. For example, ceramic tile adhesives containing clays, asbestos and long chain organic polymers are said to exhibit improved rheological properties. However, the improved rheological properties may be gained at the expense of reduced bonding strength and/or workability.
U.S. Pat. No. 4,487,864, issued on Dec. 11, 1984 to Bermudez et al., discloses modified carbohydrate polymers for use as water retention aids. The carbohydrate polymers, which may include cellulose ethers, starches such as tapioca starches and modified starches, are modified with up to 50% by weight of a water-swellable, crosslinked nonflocculating organic polymer, based on the total weight of the carbohydrate polymer and the crosslinked organic polymer. The addition of the nonflocculating polymer is said to exhibit improved rheological properties as compared to compositions containing conventional retention aids. Cellulose ethers so modified are preferably used as the water retention aid in the cementitious compositions.
U.S. Pat. No. 4,073,658, issued Feb. 14, 1978 to Ohtani et al., discloses a hydraulic cement composition comprising a hydraulic cement and a water reducing agent which is a product of hydrogenation of an oligosaccharide. The oligosaccharide is formed by the hydrolysis of starch, cellulose or hemicellulose. An aqueous solution of the hydrolyzate is prepared and the terminal aldehyde groups thereof are reduced and converted to hydroxyl groups by hydrogenation conducted under high temperature and high pressure in the presence of a catalyst.
U.S. Pat. No. 4,451,649, issued May 29, 1984 to Teubner et al., discloses hydroxypropyl-modified starch which is etherified in the presence of a small amount of water with an excess of propylene oxide. The hydroxypropyl starch is used in combination with cellulose ethers as a "set-up agent", which is said to influence the rheology of a plaster and improve its processing properties.
Japanese Public Patent Disclosure Bulletin No. 61-72663 discloses cement mortar compositions, wherein hydroxyalkyl modified starches of limited degrees of substitution are used in combination with water-soluble nonionic cellulose ethers to promote sag resistance. It is stated that other modified and unmodified starches do not have the required properties to be used in the mortar compositions.
U.S. Pat. No. 4,654,085, issued Mar. 31, 1987 to Schinski, discloses an additive for cementitious compositions which comprises a cellulose ether, a starch ether and a polyacrylamide. The additives are said to improve the anti-sag properties of the cementitious compositions.
As noted above, carbohydrate polymers, particularly cellulose ethers, are employed as water retention aids. Also as noted above, the cellulose ethers are commonly used in combination with modified starches to improve rheological properties of the cementitious compositions containing the cellulose ether water retention aids. However, the addition of these materials increases the cost of the mortar composition, creating an incentive to find more economical polymers. In addition, the use of modified starches requires additional processing of the starch prior to incorporation into the cementitious composition.
There is a long felt need to develop new water retention aids for use in cementitious and adhesive compositions which are economically viable replacements for the cellulose ethers, which do not require starch pretreatment processes, and which provide sufficient water retention properties and favorable rheological properties for good workability. In addition, the additives must maintain adhesive bond strength in the cementitious and adhesive compositions and not contribute to the loss in adhesive properties thereof.
It has now been discovered that a partial substitution of cellulose ethers can be made with an unmodified, cold-water-soluble starch, without the loss of water retention properties, or favorable rheological properties for good workability, or bond strength in the adhesive or cementitious composition. This discovery is especially surprising in view of the fact that modified starches, which are highly preferred for use in water retention aids according to the overwhelming teaching of the prior art, were found to exhibit a loss in adhesive properties when used similarly in combination with a cellulose ether. While the independent use of cellulose ethers and starches as water retention aids are known in general, combinations of cellulose ethers and unmodified, cold-water-soluble starches as water retention aids are not disclosed or taught in the prior art.